Seals within turbofan engines are conventionally disposed axially and such seals are normally one of two types: labyrinth seals or brush seals. With either type of seal, the seal is attempting to minimise the axial air flow between a rotating element (typically a shaft, disc or bladed disc and a static element (typically a casing). In the case of a labyrinth seal, the rotating element has a series of grooves and the static element has a smooth concentric surface (normally coated with an abradable substance). In the case of a brush seal, the rotating element is a smooth surface and the static element holds a continuous ring of bristles in contact with the smooth rotating surface.
Larger diameter seals often have higher leakage rates than necessary because at large diameters, labyrinth seals must frequently be set to run at large clearances, which is very detrimental to their performance. The reason for this is that there is too much relative movement between the static element and the surface of the rotating part, either from relative lateral (radial) movements or thermal differential expansion or both. It should be noted that lateral (radial) movements of the static element can be due to local deflections as well as movement of the entire static element. Furthermore, the thermal differential expansion issue is often exacerbated by having to use materials for the static and rotating elements which do not have the ideal thermal differential expansion characteristics and also because the relative masses of the elements lead to thermal differential lags. Too much relative movement is often the reason brush seals, which are often more efficient than labyrinth seals, are not suitable at many locations within a turbofan, meaning labyrinth seals must be used.
The present invention therefore seeks to address these issues.